Hollow articles can be made from foamed thermoplastic material by die extrusion and such articles may weigh substantially less than equivalent non-foamed plastic articles. This saving of material can result in lower manufacturing costs provided that the foamed plastic articles satisfy all specifications for commercially salable items. Among such specifications are smoothness and gloss, particularly in connection with the inside surface of tubular articles prepared from foamed thermoplastic material.
Exit face die restrictions for extrusion of thermoplastic materials have been used in the past to increase the pressure drop across the exit face of the die, c.f., U.S. Pat. No. 3,427,371 and in particular U.S. Pat. No. 3,502,753. The U.S. Pat. No. 3,427,371 describes die restrictions for producing solid extrudates and the U.S. Pat. No. 3,502,753 uses die restrictions to keep the bubble size small in foamed thermoplastic parisons to improve the quality of the article blow-molded therefrom. The problem to which the instant invention is addressed is one related to the inside wall of a foamed tubular article. In particular, the problem was to produce a foamed tubular article of a density dictated only by the mechanical properties desired which, in addition and importantly for commerical usage, has as smooth an interior surface as possible and, additionally, if the thermoplastic material permits, as high a gloss on said interior surface as possible.
Now it has been found that the addition of certain properly shaped restrictions to the exit face of the extrusion die produces foamed tubular articles having substantially smoother inside surfaces than previously and, with certain thermoplastic materials, additionally, a higher gloss to said inside surfaces.